The present invention relates to a circuit arrangement for measuring the rotational speed of a generator. More particularly, it relates to a circuit arrangement for a starting detection of a three-phase generator.
In motor vehicles, three-phase generators which excite themselves after the motor driving them has started up are usually used to generate the electrical energy. So that this self-excitation occurs reliably and safely, measures which improve the self-excitation are usually initiated directly after the motor vehicle engine starts.
One customary method consists in using the charge monitoring lamp to pre-excite the generator. For this purpose, during starting, energy is fed to the exciter winding from the battery via the charge monitoring lamp. Another method consists in providing an additional lead between the battery and the exciter winding of the generator and conducting an additional exciter current via this connection during the starting process.
The aforesaid measures are disclosed, for example, in the German Offenlegungsschrift 38 43 161 and the corresponding U.S. Pat. No. 5,107,198. In the known solutions, additional means for detecting the start of the engine are required.
However, in generator voltage controller systems, it is also known to use the rotational-speed-dependent alternating voltage in a phase winding of the three-phase generator in order to detect starting. In such a case, a voltage measurement is carried out in the voltage controller, during which voltage measurement the voltage is measured between the phase and earth and, when a threshold value is exceeded, it is detected that the engine is starting and thus the three-phase generator is beginning to rotate.
This type of starting detection has the disadvantage that, if the switch-on threshold is too high, the cut-in speed of the generator is too high and, if the threshold is too low, the signal-to-noise ratio becomes too small, so that starting may be detected erroneously.